1. Field of the Invention
The present invention relates to a rotary drive apparatus for a disk, such as, for example, a microfloppy disk and more particularly is directed to a rotary drive apparatus of the type in which, the disk is mounted on a turntable and automatically centered in respect to the latter.
2. Description of the Prior Art
A conventional rotary drive apparatus for a microfloppy disk (to be referred to as a disk hereinafter) will first be described with reference to FIGS. 1 to 3.
As is known, a disk 1 is rotatably housed inside a disk cassette 2 comprising upper and a lower halves 2a and 2b. A central hole 4 and a drive hole 5 are formed respectively at the center and at a position spaced radially therefrom in a center plate 3 which is made of metal and fixed at the center of the disk 1. The central hole 4 has a square shape, and the drive hole 5 has a rectangular shape. The center plate 3 is loosely fitted into a turntable receiving hole 6 formed at the center of the lower half 2b. A pair of upper and lower pad/head receiving holes 7 (FIG. 1) are formed respectively in the upper and lower halves 2a and 2b so as to extend along the radial direction of the disk 1 and are opened/closed by a shutter 8.
A central pin 11 and a drive pin 12 are arranged respectively at the center and at a position spaced radially therefrom on a turntable 10 of a disk player (FIG. 2). The central pin 11 is disposed at the upper end of a motor shaft 13 which is driven by a motor (not shown) arranged below the turntable 10. A ball bearing 14 is mounted on the upper end of the drive pin 12. The drive pin 12 is perpendicularly fixed to a pin mounting portion 15a of a leaf spring 15 of a substantially annular shape. The leaf spring 15 is fastened to the turntable 10 with a plurality of screws 17 through a spacer 16. A magnetic chuck 20 above the leaf spring 15 comprises a magnet plate 18 and a yoke plate 19 and is also fastened to the turntable 10 by the screws 17. A smooth sheet 21 of a nonmagnetic material is fixed onto the magnet plate 18. The upper end of the drive pin 12 with the ball bearing 14 thereon projects upward through a hole 22 formed in the magnet plate 18 and the yoke plate 19.
The torsion of the pin mounting portion 15a of the leaf spring 15 allows the drive pin 12 to pivot in the radial direction of the turntable 10 as indicated by arrows a and a'.
In a disk player, the shutter 8 is open, as shown in FIG. 1, and the disk 1 in the disk cassette 2 is horizontally mounted on the turntable 10. When the disk 1 is mounted in this manner, a pad 24 and a recording/reproducing head 25 are inserted into the pad/head receiving holes 7 of the disk cassette 2. The disk 1 is elastically pressed against the recording/reproducing head 25 by the pad 24. Meanwhile, as shown in FIGS. 2 and 3, when the disk 1 is mounted, the central pin 11 and the drive pin 12 of the turntable 10 are inserted respectively into the central hole 4 and the drive hole 5 of the center plate 3, and the center plate 3 is attracted by the magnetic chuck 20 while being maintained horizontally on the smooth sheet 21.
After the disk 1 is mounted in this manner, the turntable 10 is driven by the motor in the direction of arrow b on FIGS. 1 and 3. The drive pin 12 acts against an end edge 5a of the drive hole 5 of the center plate 3 which is at its front side with respect to the rotating direction of the turntable 10, and thereby the disk is rotated in the direction of arrow b. At the same time, the recording/reproducing head 25 is moved in the radial direciton (indicated by arrows c and c' in FIG. 1) so as to perform recording or reproduction of data on or from the disk 1.
In this conventional apparatus, at the initiation of the driving of the disk 1, when the turntable 10 is driven in the direction of arrow b after the drive pin 12 has been inserted in the rear portion of the drive hole 5 with respect to the rotating direciton of the turntable 10, as indicated in alternate long and short dashed lines in FIG. 3, the drive pin 12 is moved in the direction of arrow b within the drive holes 5 and is pressed against the front end edge 5a with respect to the rotating direction of the turntable 10 and also pressed against the outer side edge 5b in a direction toward the outer periphery of the turntable 10, as indicated by the solid line in FIG. 3. During this operation, the torsion of the pin mounting portion 15a of the leaf spring 15 causes the drive pin 12 to incline in the direction of arrow a' from the upright state indicated by the alternate long and short dashed line to the inclined state indicated by the solid line in FIG. 2. Then, the force of the drive pin 12 in the direction of arrow a owing to the reactive torque of the pin mounting portion 15a of the leaf spring 15 serves to press the drive pin 12 against the side 5b of the drive hole 5. As a result, the center plate 3 of the disk 1 is moved in the direction of arrow a shown in FIG. 3, so that two sides 4a and 4b of the central hole 4 of the center plate 3 are pressed against the side surface of the central pin 11 of the turntable 10. Thereby, the center line of the disk 1 is aligned with that of the turntable 10.
As shown in FIG. 3, a rotary drive force F1 and a biasing force F2 act on the drive hole 5. The rotary drive force F1 acts against the end edge 5a to rotate the disk 1 in the rotating direction of the turntable 10 (arrow b). The biasing force F2 acts to press the pin 12 against the side edge 5b and bias the drive hole 5 in a direction away from the center of the turntable 10. As a result, reactive forces F3 and F4 are generated on the central pin 11. A resultant force F5 of the reactive forces F3 and F4 serves to press the two sides 4a and 4b of the central hole 4 against the side surface of the central pin 11 and to align the center line of the disk 1 with that of the turntable 10. A diagonal line P of the central hole 4 inclines about 30.degree. in respect to the end edge 5a of the drive hole 5, so that when the forces F1 (=F3) and F2 (=F4) correspond to design values therefor, the line of action of the resultant forces F5 coincides substantially with the diagonal line P of the central hole 4.
However, the conventional apparatus having the above-mentioned construction has the following problems.
Firstly, the force F2 is determined by the torsion of the pin mounting portion 15a of the leaf spring 15. Therefore, the upper limit of the force F2 is limited by the characteristics of the pin mounting portion 15a of the spring.
In contrast to this, the rotary drive force F1 is increased in proportion to the torque loaded on the disk, which mainly depends upon friction loads between the pad 24, the recording/reproducing head 25 and the disk 1.
The rotary drive force F1 and the torque T are given by the following equation: EQU T=F1.times.R1 EQU F1=T/R1
where R1 is the radial distance of the drive pin 12 from the axis of central pin 11.
This means that the rotary drive force F1 increases in proportion to an increase of the torque T loaded on the disk 1.
When the reactive force F3 increases with an increase in the rotary drive force F1 while the biasing force F2 is kept constant, as indicated by the alternate long and short dashed lines in FIG. 3, the line of action of the resultant force F5 is substantially deviated from the diagonal line P of the central hole 4 in the direction toward side 4b.
In the conventional apparatus described above, there is an upper limit to the torque that may be loaded on the disk 1. In other words, when the torque is exessively increased, the biasing force F2 generated by the leaf spring 15 cannot correctly urge the central pin 11 toward the side 4a of the central hole 4. Therefore, as indicated by the alternate long and short dashed lines in FIG. 3, the side 4a of the central hole 4 floats away from the side surface of the central hole 11. As a consequence, the center line of the disk 1 cannot be correctly aligned with that of the turntable 10.
Since the rotary drive force F1 and the biasing force F2 have a correlation in aligning the center line of the disk 1 with that of the turntable 10, and since the characteristics of the leaf spring 15 for producing the biasing force F2 greatly affect the alignment performance of the disk 1, the characteristics of the leaf spring 15 and the sizes of the respective parts must be controlled with high precision, resulting in an increase in the cost of the overall apparatus.
In the above-described apparatus, in order to align the center line of the disk 1 with that of the turntable 10, the upper limit of the torque loaded on the disk 1 must be limited. For this reason, the frictional force acting between the disk 1 and the pad 24 or the recording/reproducing head 25 must be adjusted with high precision. In addition, the frictional force acting on the drive pin 12 upon contact with the edges 5a and 5b of the drive hole 5 must be kept at a minimum. Because of this, in the conventional apparatus, an expensive ball bearing 14 is mounted on the upper end of the drive pin 12 so as to be able to press the drive pin 12 against the edges 5a and 5b of the drive hole 5 through the ball bearing 14. However, this also increases the cost of the apparatus.
Furthermore, in the above-mentioned conventional apparatus, when the biasing force F2 is generated for pressing the ball bearing 14 of the drive pin 12 against the side edge 5b of the drive hole 5 in the direction of arrow a in FIG. 3, the drive pin 12 is inclined as indicated by the full lines in FIG. 2. For this reason, the center plate 3 placed horizontally on the turntable 10 is affected by the inclination of the drive pin 12 and may be inclined with respect to the turntable 10.